Grimbsy Town boss David Artell described Manchester United as an "unbelievable team" after his side's giant killing on Wednesday night.
United entered the Carabao Cup second round due to their failure to qualify for Europe last season, but they were heavy favourites to progress to the next round against Grimsby.
Grimsby did not read the script and had a two-goal advantage by half-time. Ruben Amorim looked shell-shocked at the performance from his team and made a triple change for the second half, which helped United get back into the game.
Bryan Mbeumo was introduced from the bench and scored to make it 2-1, while Mason Mount delivered the ball into the box for Harry Maguire's late equaliser to force a penalty shootout.
However, Grimsby prevailed in the shootout to inflict an embarrassing defeat on United.
"A long penalty shootout," said Grimsby boss Artell during his interview with ITV Sport. "I said halfway through the shootout, first round last year we beat Bradford 9-8 on penalties. I said before the penalty shootout started, look, what's a penalty shootout?
"It's a technical thing, it's a psychological thing. You've got brilliant techniques. We work hard every day to get better. It's a thing above the front door.
"Another chance to get better and this group want to get better. The techniques are fine, they're going to feel nervous walking up, butterflies. Don't worry about that.
"This is an accumulation of 15 months, 18 months, all encompassed in one night. Look, they're an unbelievable team, Manchester United. They've got an unbelievable manager and they're on the right track, you can see that.
"We've got an unbelievable group of players as well, who want to play for this football club, live in the area and want to get better every day."